1. Field of the Invention
The present invention relates generally to a diagnostic arrangement which enables a sensor malfunction or the like to be detected, and more specifically to such a type of system which enables the standard against which the instant performance is judged, to be varied in accordance with the number of times a given parameter is sampled.
2. Description of the Prior Art
In order to meet recent stringent emission control regulations it is becoming necessary to equip vehicles with diagnostic arrangements which enable the malfunction of sensors or systems associated with the control/operation of an exhaust gas recirculating system generally abbreviated as an EGR, air-fuel ratio control systems and the like, to be detected and a warning suitably issued/generated.
JP-A-62-51746 discloses an arrangement which has been proposed in order to monitor the operation of an EGR system and to diagnose improper operation thereof. In this arrangement, a valve which controls the flow of exhaust gas from the exhaust conduit to the induction system, is momentarily closed and the induction pressure which is detected during this period, is compared with the pressure detected while the valve is open. This data is used to develop a pressure differential value .DELTA.P. That is to say .DELTA.P=(Pon-Poff) where Pon and Poff represent the pressures which are detected while the valve is open and closed, respectively.
While this pressure differential exhibits a predetermined relationship with a predetermined standard value, the operation of the valve is deemed to be normal.
However, with this type of arrangement the parameter on which the decision is based (in this case the pressure differential value) is apt to vary to a degree that erroneous diagnosis is apt to occur.
That is to say, FIGS. 1 to 3 show the situations wherein the number of time the pressure differential is sampled once, twice and a number of times greater than 1. As will be appreciated from FIG. 2, when only one sampling is taken, the abnormal (malfunction) and normal operation pressure differential distributions overlap to degree that it is possible that, even though the arrangement is working normally, still the pressure differential which is produced under certain engine operating conditions can be taken as being indicative of a malfunction and vice versa.
As a result of the above, when a predetermined standard pressure differential value is used as the .DELTA.PNG, due to the above distribution overlap, it is possible that even if the system is operating normally, still a malfunction indication can be undesirably produced. While reduced, this possibility remains even when two samples are taken (see FIG. 2).
To overcome this problem and to increase the accuracy of the diagnostic system, it has been thought to record a number of pressure differential values and derive an average .DELTA.Pav which can then be compared with a predetermined standard value .DELTA.PNG. In the event that the deviation exhibits a normal distribution, the standard deviation .sigma.A exhibits the following relationship with the number of samples n which are used to develop the average. ##EQU1##
As the value of n increases the average .DELTA.Pav is such that the normal and abnormal operation distributions separate in the manner depicted in FIG. 3 wherein the possibility of error with respect to .DELTA.PNG becomes essentially non-existent.
However, with this type of arrangement it is very difficult to achieve the required number of sequential pressure differential samples. That is to say, during vehicle operation the time for which the engine operational characteristics remain in a region suitable for sampling and system diagnosis is possible, is relatively short. Accordingly, if n is set to a suitably high value, the possibility that the required number of samples which are needed to develop a fully reliable .DELTA.Pav value, will not be taken during any given run of the diagnostic routine exists and, leads to the drawback that it may be some time before a complete diagnosis can be successfully carried out and the existence of a malfunction actually determined.